// Copyright 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "base/device/base/synchronization/one_writer_seqlock.h"

namespace device {

OneWriterSeqLock::OneWriterSeqLock()
    : sequence_(0)
{
}

base::subtle::Atomic32 OneWriterSeqLock::ReadBegin() const
{
    base::subtle::Atomic32 version;
    for (;;) {
        version = base::subtle::NoBarrier_Load(&sequence_);

        // If the counter is even, then the associated data might be in a
        // consistent state, so we can try to read.
        if ((version & 1) == 0)
            break;

        // Otherwise, the writer is in the middle of an update. Retry the read.
        base::PlatformThread::YieldCurrentThread();
    }
    return version;
}

bool OneWriterSeqLock::ReadRetry(base::subtle::Atomic32 version) const
{
    // If the sequence number was updated then a read should be re-attempted.
    // -- Load fence, read membarrier
    return base::subtle::Release_Load(&sequence_) != version;
}

void OneWriterSeqLock::TryRead(bool* can_read,
    base::subtle::Atomic32* version) const
{
    DCHECK(can_read);
    DCHECK(version);

    *version = base::subtle::NoBarrier_Load(&sequence_);
    // If the counter is even, then the associated data might be in a
    // consistent state, so we can try to read.
    *can_read = (*version & 1) == 0;
}

void OneWriterSeqLock::WriteBegin()
{
    // Increment the sequence number to odd to indicate the beginning of a write
    // update.
    base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
    // -- Store fence, write membarrier
}

void OneWriterSeqLock::WriteEnd()
{
    // Increment the sequence to an even number to indicate the completion of
    // a write update.
    // -- Store fence, write membarrier
    base::subtle::Barrier_AtomicIncrement(&sequence_, 1);
}

} // namespace device
